Process and device for producing high-viscosity or highly stabilized reaction-stable polyamides, and for continuously demonomerizing polyamides

ABSTRACT

The invention concerns a process and device for producing high-viscosity or highly stabilized, reaction-stable polyamides, in particular polyamide 6, with reactive amino end groups of less than 50 μeq/g and for continuously demonomerizing polyamides. The object of the invention is to produce simply, reliably and economically a high-viscosity or highly stabilized reaction-stable polyamide, in particular polyamide 6, and if necessary to carry out continuous demonomerization. According to the invention, the molten polyamide is brought into intimate contact, or intimately mixed with preheated nitrogen in a reaction system in at least one stage and a specific dwell time is set for the melt in a liquid phase. During the production of polyamide 6, expelled lactam is processed and fed to a polymerization reactor.

BACKGROUND OF THE INVENTION

The invention relates to a process and the device for carrying out theprocess for producing high-viscosity or highly stabilized,reaction-stable polyamides, in particular polyamide 6, with reactiveamino end groups of less than 50 μeq/g and for continuouslydemonomerizing polyamides.

High-viscosity polyamide can be produced batchwise in autoclaves andcontinuously in polymerization reactors. However, only a maximumsolution viscosity of 3.4 can be achieved by means of these processes(solution viscosity based on polymer-sulfuric acid solution containing 1g of polyamide in 100 ml of 96% strength acid). However, there are alsoprocesses in which polyamide granules are subjected to postcondensationbelow the polyamide melting point with nitrogen at temperatures in therange of 140-190° C. and with dwell times of 20-60 h. These processeshave the disadvantage that the establishment of high viscosities takes avery long time and such processes are therefore uneconomical. Patent PE3923061 describes a process of this type for polyamide 6postcondensation, in which the polyamide granules are treated in aheating zone over 35 h.

It is known that, during the polycondensation of ε-caprolactam to give apolyamide at the reaction temperature of 240-280° C. usually used inproduction, an equilibrium which results in a water-soluble fraction ofabout 10-13% is established. The equilibrium is temperature-dependent;the monomer content increases with increasing temperature. Copolyamides,one of whose starting materials is ε-caprolactam, likewise have amonomer content in chemical equilibrium. For the processing of thegranules, it is necessary to reduce the monomer content. The followingmethods are available for this purpose:

extraction of the granules with hot water and those for

demonomerizing the melt in vacuo.

The aqueous extraction predominantly used today is technicallycomplicated and expensive since it is necessary to carry out the stepsof spinning and cooling of the polyamide 6 melt in tape form,comminuting of the tapes to give granules, extraction of the granules,drying of the granules and reconversion of the granules to the moltenstate. This method of extraction with hot water can be carried outbatchwise and continuously. It is used on a large industrial scale byall polyamide 6 producers.

In contrast, the route via the granules can be dispensed with in vacuumdemonomerization. The melt arriving from the polycondensation reactorpasses continuously through the demonomerization stage and is then feddirectly to the spinning means.

U.S. Pat. No. 3,578,640 describes a process for demonomerizing the PAmelt by means of a vacuum. In this process, after the vacuum stage, avery complicated technical procedure is carried out for discharging themelt and for the downstream finisher for further reducing the content ofextractables. In this technique, for example, shaft seals inevitablylead to leaks on the vacuum system and associated oxidative damage tothe melt by atmospheric oxygen which has penetrated. The oxidativedamage cannot be suppressed even by feeding in inert gases, inparticular nitrogen.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a process and a device forcarrying out the process of the type mentioned at the outset, whichpermits simple production of high-viscosity or highly stabilized,reaction-stable polyamides and, if necessary, demonomerization. Theprocess starts from a melt which is originally virtually in chemicalequilibrium. It is intended to produce a polymer which, is not inchemical equilibrium but which owing to the very low content of aminoand carboxyl end groups, is very stable and thus scarcely undergoes anypostcondensation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically sets forth a plan for carrying out the process andthe device of the invention.

FIG. 2 shows a reaction container of a stage with a device for directintroduction of nitrogen into a melt phase.

FIG. 3 shows another device used in the present invention.

DESCRIPTION OF THE INVENTION

This object is achieved by the features stated in patent claim 1.Advantageous embodiments and further developments of the invention areevident from the subclaims.

According to the invention, the melt is introduced continuously into thereaction container and is brought into intimate contact with, and/orintimately mixed with, pre-heated nitrogen, and a defined dwell time isset for the melt in a liquid phase in the lower part of the reactioncontainer before said melt is continuously discharged, expelledsubstances being collected via pipelines and, particularly in theproduction of polyamide 6, expelled caprolactam being introduced viapipelines into a collecting container and being fed to a polymerizationstage with continuous circulation.

In the reaction container, the polyamide melt is passed oversurface-increasing and simultaneously surface-regenerating elementsand/or into a melt phase by one or more horizontal separation meanshaving passages, and is treated with preheated nitrogen.

The nitrogen preferably preheated to 150-300° C. before its entry intothe reaction container, is introduced into the reaction container abovethe level of the melt phase present in the lower part of the reactioncontainer and—where present—below a separation means. In the reactioncontainer, the nitrogen is forced through a melt layer present above aseparation means having passages.

The melt subjected to the reaction is fed to the liquid phase in thelower part of the reaction container via at least one overflow pipewhich extends vertically through the separation means into the meltphase at the bottom of the reaction container. If surface-increasingelements are additionally used, they are treated with nitrogen after itsemergence from the melt layer.

During the production of high viscosities and stabilities and, ifnecessary, the demonomerization, a melt phase is established forhomogenization of the melt in every stage; the dwell time of the melt inthe liquid phase is ≦30 min. The homogenization carried out in thismanner has the advantage that differences in the molecular weight and inthe residual extract between the edge and middle zone of the flowingmelt are kept small. Such differences are to be expected owing to thepostcondensation taking place over a period of time in combination withthe flow profile, depending on the extent to which the polycondensationequilibrium was disturbed by the feeding in of nitrogen.

Depending on the parameters of the polymer to be produced, the polyamidemelt is treated according to the invention in succession in one or morestages in reaction containers and is removed continuously from the lowerpart of the reaction container and introduced into the reactioncontainer following the stage. After emergence of the melt from the laststage of the reaction container, the melt is cooled by means of a heatexchanger and the heat recovered is used for preheating the monomericstarting material, for example for preheating the caprolactam in thecase of polyamide 6 melt. The caprolactam expelled in the treatment ofpolyamide 6 melt with nitrogen is fed to the collecting container in theupper region of the reaction container via a jacketed pipeline.

Since, in the treatment of polyamide 6 melt or copolyamide, one of whosestarting materials was caprolactam, oligomers which may block thepipelines to the collecting container are expelled in addition tocaprolactam, the material collected in the collecting container ispreferably continuously circulated via a further pipeline through thepipelines leading from the individual stages to the collectingcontainer. The pipelines are preferably heated, the resultingcaprolactam/oligomer mixture being heated to a temperature such that theoligomers are dissolved in the caprolactam.

The expelled caprolactam and the simultaneously expelled oligomer can beprocessed and fed as part of the starting material to the polymerizationplant.

In the case of the process according to the invention and the device forcarrying out the process, the chemical equilibrium of the starting meltis continuously and deliberately disturbed on entry into the firstreaction stage by the treatment with nitrogen.

In subsequent reaction stages, the establishment of an equilibrium ofthe melt is prevented by further treatment of 4.5 g of polyamide in 100ml of distilled water, refluxed for 10 h at 100° C.).

Compared with the known polymerization processes, a substantially moreadvantageous space-time yield is achieved with the process developed.

In embodiment 1 of the invention, the production of high-viscosityspinnable polyamide 6 is illustrated in more detail below with referenceto a device for carrying out the process:

The drawing in FIG. 1 shows the embodiment of a plant for carrying outthe process and the device.

The plant described consists of three stages 1 a, 1 b and 1 c. In eachof the stages 1 a to 1 c, the melt is passed continuously into areaction container via surface-increasing elements 2, and in each stagenitrogen is introduced in the temperature range 250-280° C. via apreheater 5. In stage 1 a, a melt in the equilibrium state and at atemperature of 280° C. is fed in, it being possible for said melt to bea water-rich prepolymer from a pressure stage or a polymer from apostcondensation stage.

In each stage, caprolactam, oligomers and the water formed by acondensation reaction are expelled by the nitrogen fed in, the watercontained in the chemical poly-condensation equilibrium additionallybeing expelled in stage 1 a when a water-rich prepolymer is fed in.Caprolactam, such a melt with nitrogen, the water formed in the phaseinvolving the formation of amido groups (—NHCO—) by condensation oflinear polymers being continuously expelled and hence a high reactionrate being achieved.

The treatment of the polyamide melt with dry preheated nitrogen resultsin a considerable viscosity increase which makes it possible to producea polyamide melt having a solution viscosity of more than 3.8. In order,if necessary, to reduce the viscosity, which is greatly increased by thetreatment with dry nitrogen, or to set said viscosity to a desiredvalue, it is intended, if necessary, to supply super-heated steam inaddition to the nitrogen in one or more stages in order to moisten thenitrogen. If it is intended to produce a low-viscosity polyamide melt, achain stabilizer is added to the starting material at the beginning ofthe polymerization process. A highly stabilized polymer exhibiting verylittle postcondensation is formed.

Owing to the small number of reactive amino end groups, the meltobtained and not in chemical equilibrium exhibits such littlepostcondensation that further processing to give films, filaments, etc.is not hindered.

Surprisingly, it was found that the content of extractables in thepolymeric melt is reduced even to ≦1.6% by the process according to theinvention and the device for carrying out the process (content ofextractables based on oligomers, water and nitrogen are discharged intoa collecting container 7 via a pipeline 6 opening into the upper regionof the respective stage.

The total reactor system is sealed off from the surrounding air by animmersion means 12. The surface-increasing elements 2 are formed in sucha way that continuous surface regeneration occurs during the passage ofthe melt over these elements. In the example, expanded metal sheet isused for this purpose. Since the melt tends to form drops on thesurface-increasing elements 2 above a limited length of the element andrandom distributor devices counteract this drop formation only to aslight extent, the treatment is carried out in several stages (1 a, 1 b,1 c).

After each stage, the melt is passed by a melt pump as discharge means 4uniformly over the total width of the surface-increasing elements 2 ofthe reaction container of the subsequent stage.

In each process stage, a small melt phase is established forhomogenization of the melt; the dwell time of the melt in this liquidphase is ≦30 min. The total dwell time of the melt in the stages 1 a, 1b and 1 c is about 2 h, taking into account the passage times.

Heat is removed from the polyamide 6 melt by the evaporation of waterand by vaporizing caprolactam. The heating of the melt to 280° C. forthe next stage is effected in this example in the melt phase and in thedownstream discharge device, consisting of slide valve 3 and melt pump4. The melt is discharged continuously from each stage.

After the melt has been discharged from the last stage 1 c, it is cooledin the heat exchanger 15 with diphyl from about 280° C. to about 245° C.The heat recovered is used, by means of a further heat exchanger notshown, for preheating the monomeric intermediate, in this caseε-caprolactam, before entry into the polymerization reactor.

The monomeric caprolactam expelled from each reaction container 1 a, 1b, 1 c and collected via the pipeline 6 contains oligomers which mayblock the pipeline. For this reason, in addition to heating of thepipeline 6, a circulation pipeline 8 which is likewise heated and is fedvia a pump 9 from the collection container 7 is provided. As a result ofheating the pipelines, the resulting caprolactam/oligomer mixture isfurther heated to a temperature such that the troublesome oligomers aredissolved in the caprolactam.

In the circulation pipeline 8, a pressure of 7 bar (gauge pressure) isestablished after the pump 9 up to the control valve 13. By means ofthis pressure, the caprolactam/oligomer mixture obtained in thecollecting container 7 is transported as a part-stream via the controlvalve 14 into the pressure stage of the polymerization plant to thecaprolactam, the control valve 14 operating as a function of the levelin the collecting container 7.

If water-enriched prepolymer is fed into the device according to theinvention, this water is removed from the caprolactam/oligomer mixturein the collecting container 7 via a rectifier 10. The steam emergingfrom the rectifier 10 is condensed in the heat exchanger 11.

In the treatment of the polyamide melt with dry nitrogen, a considerableviscosity increase occurs. If this viscosity increase is too large or ifit is desired exactly to adjust the viscosity of the material emergingat the outlet of the last stage, steam is introduced in one or morestages together with nitrogen or alone, as described with reference tostage 1 c.

Without the supply of steam, it is possible to produce a high-viscosity,stable polyamide 6 melt having a solution viscosity of 3.8 and an aminoend group concentration of 25 μeq/q and a content of extractables of2.5%. Viscosity control in the range of 3.2-3.8 is possible by passingthe steam/nitrogen mixture into the last process stage.

Embodiment 2 illustrates a device for carrying out the process, whichstarts from the arrangement shown in embodiment 1.

The preheated nitrogen is fed only into the lower-most stage 1 c,whereupon nitrogen then flows through the stages 1 b, 1 a above in asort of series circuit, supplementation with fresh nitrogen additionallybeing effected in each stage. The resultingcaprolactam/oligomer/nitrogen mixture is then removed only from stage 1a and fed to the collecting container 7.

Embodiment 3 of the invention describes the production of a spinnable,low-viscosity polyamide 6 melt, in which the arrangement shown inembodiment 1 is used as a starting point.

If it is intended to produce a polyamide 6 melt having a solutionviscosity of 2.4 and different amino end group concentration, there areseveral possibilities to do so if acetic acid is used as a stabilizer.

1. Stabilization of the reactor mixture at the beginning of thepolymerization process with 0.34% of acetic acid. In each stage,preheated dry nitrogen at 280° C. is fed in. This polymer has an aminoend group concentration of 6 μeq/g.

2. Stabilization of the reaction mixture at the beginning of thepolymerization process with 0.18% of acetic acid. Preheated dry nitrogenat 280° C. is fed into each stage. This polymer has an amino end groupconcentration of 31 μeq/g.

3. Stabilization of the reaction mixture at the beginning of thepolymerization process with 0.30% of acetic acid. Preheated dry nitrogenat 280° C. is fed into the stages 1 a and 1 b and preheated dry steam at280° C. into the stage 1 c.

This polymer has an amino end group concentration of 11 μeq/g. When drypreheated nitrogen or steam or a combination of the two gases is fed in,a total extract of ≦1.8% is established by the expulsion of caprolactamand oligomers from the polyamide 6 melt. In order to avoid troublesomereformation of extractables in the melt, the dwell time of the melt inthe liquid phase before each melt pump may be not be more than 30 min.This melt is suitable for the production of filaments by the POY and FDYprocess.

Embodiment 4 describes the production of a spinnable, high-viscositypolyamide 6 melt. The production is carried out in principle asdescribed in embodiment 1, except that two reactor stages 1 a and 1 bare used, the dewatering of the melt shown in FIG. 2 additionally beingcarried out in stage 1 a.

FIG. 2 shows the reaction container of a stage with a device for directintroduction of nitrogen into a melt phase in combination withsurface-increasing elements from FIG. 1.

The polyamide 6 melt flows over the surface-increasing element 2 into amelt layer which is adjusted by the separation means having passages 17,for example a perforated plate.

The height of the melt layer is determined by the height of the overflowpipe 18. Nitrogen is passed via the preheater 5 under the separationmeans having passages 17 and directly through the polyamide 6 meltlayer. The nitrogen is then further transported on the melt film of thesurface-increasing element 2 and, as already described in embodiment 1,is further treated together with the expelled water, the caprolactam andthe oligomers.

The polyamide 6 melt running out of the melt layer through the overflowpipe 18 enters the melt phase which is positioned underneath and inwhich the overflow pipe 18 extends close to the bottom. In order toestablish a uniform dwell time of this polyamide 6 melt in the lowermelt phase, the melt is passed upward again through a container 19 whichis open at the top and is provided with an outlet hole. The outlet holeis a small hole which ensures the melt immersion tank 19 is emptied onrunning down the polymerization plant. By the discharge means 4, forexample a melt pump, the polyamide 6 melt is introduced into thereaction stage underneath or fed for granulation or directly furtherprocessed. A polymer having a solution viscosity of 3.65 and a contentof extractables of 2.5% is formed.

This additional dewatering and demonomerization described can be used inone or more reaction containers of stages 1 a and 1 b for thedemonomerization of the melt and for the production of a high-viscositymelt.

Embodiment 5 describes the production of a spinnable polyamide 6 melt ofnormal viscosity. The production is carried out in principle accordingto embodiment 4. Before the beginning of the polymerization process, thereaction mixture is stabilized with 0.12% of acetic acid. Instead ofnitrogen, preheated dry steam at 280° C. is fed in. The polymer producedhas a solution viscosity of 2.4 and an amino end group concentration of41 μeq/g and a content of extractables of 2.5%.

Embodiment 6 describes the production of a spinnable, high-viscositypolyamide 6 melt. The drawing in FIG. 3 shows the embodiment of a plantfor carrying out the process and the device. A polyamide 6 melt in theequilibrium state is introduced via the melt distributor 16 into themelt layer underneath, which is adjusted by the separation means havingpassages 17, for example a perforated plate. The height of the meltlayer is determined by the height of the overflow pipe 18. The polyamide6 melt running out of the melt layer through the overflow pipe 18 entersa melt immersion tank 19 underneath, which has a small hole for emptyingresidues at the lowermost point. The overflowing melt from the meltimmersion tank 19 runs down the container wall onto a melt distributorand from there into the melt layer underneath. While the melt runs downthe container wall and on the distributor, separation of the nitrogenfrom the melt takes place.

From the lower melt layer, via the separation means having passages 17,the melt likewise runs via the discharge pipe 18 into the melt immersiontank 19 and over its container wall into the melt phase at the bottom ofthe reaction container. Nitrogen is forced via the preheater 9, belowthe lowest separation means 17, through the polyamide 6 melt and thenflows through the melt layers above.

Caprolactam, oligomers and water are removed from the polyamide 6 melt.This mixture leaves the reaction apparatus via the pipeline 6 and isfurther treated as described in embodiment 1. After the discharge of themelt from this reactor, it is cooled in the heat exchanger 15, asdescribed in Example 1.

The number of individual stages in this reactor can be increased inorder to establish a higher viscosity and/or to establish a lowercontent of extractables in the polyamide 6 melt.

What is claimed is:
 1. A process for producing high-viscosity or highly stabilized polyamides with reactive amino end groups of <50 μeq/g and for continuously demonomerizing polyamides by subjecting a melt which is in chemical equilibrium to postcondensation by removing the low molecular weight polycondensate, wherein the melt is introduced continuously into a reaction container and is brought into intimate contact with, or intimately mixed with, preheated, dry nitrogen, and a defined dwell time is set forth for the melt in a liquid phase in the lower part of the reaction container before continuous discharge, expelled substances being collected via pipelines and particularly in the production of polyamide 6, expelled caprolactam being introduced into a collecting container, continuously circulated and processed and fed to a polymerization reactor.
 2. The process as claimed in claim 1, wherein the polyamide melt in the reaction container is passed over surface-increasing and simultaneously continuously surface-regenerating elements and is treated with nitrogen.
 3. The process as claimed in claim 1, wherein the polyamide melt in the reaction container is passed into a melt layer which is formed above a separator having passages, nitrogen being forced through this melt layer below the separator and the melt being fed via a discharge means, which extends vertically through the separator into the melt phase at the bottom of the reaction container, to the liquid phase in the lower part of the reaction container.
 4. The process as claimed in claim 1, wherein the polyamide melt is passed over surface-increasing elements in a melt layer formed over the separator having passages.
 5. The process as claimed in any of claim 1, wherein the nitrogen is preheated to temperatures up to 300° C. before its entry into the reaction container, above the level of the melt phase present in the lower part of the reaction container.
 6. The process as claimed in any of claim 1, wherein the melt phase is maintained in the lower part of the reaction container and the dwell time of the melt in the liquid phase is set at ≦30 min.
 7. The process as claimed in any of claim 1, wherein the polyamide melt is treated in succession in a plurality of stages in reaction containers and is removed continuously from the lower part of the reaction container and introduced into the reaction container following the stage.
 8. The process as claimed in claim 5, wherein the preheated nitrogen is fed into the lowermost stage and flows therefrom through the stages above.
 9. The process as claimed in any of claim 5, wherein preheated nitrogen is fed into each stage above the lowermost stage.
 10. The process as claimed in claim 5, wherein steam is fed to at least one stage, in addition to the nitrogen.
 11. The process as claimed in claim 1 and 7, wherein caprolactam or oligomeric caprolactam or both expelled in the upper part of the reaction container are fed via pipelines to a collecting container.
 12. The process as claimed in claim 11, wherein the pipelines transporting the caprolactam or the oligomeric caprolactam or both are heated and wherein the caprolactam or the oligomeric caprolactam collected in the collecting container are continuously circulated in these pipelines.
 13. The process as claimed in any of claim 1, wherein both the individual stages and the collecting container for the caprolactam or the oligomeric caprolactam or both are sealed off from the surrounding air.
 14. The process as claimed in claim 1, wherein the excess water in the expelled caprolactam or both in the oligomeric caprolactam is separated off by means of a rectifier and is removed.
 15. The process as claimed in claim 11, wherein the caprolactam or oligomeric caprolactam or both collected in the collecting container are fed to a polymerization reactor.
 16. A device for carrying out the process as claimed in claim 1, wherein the device comprises a reaction container (1 a) through which polyamide melt flows and which has a horizontal separator having passages (17) and at least one overflow pipe (18) which passes through the separator and opens, in the vicinity of the bottom of the reaction container, into a container (19) open at the top, or the reaction container (1 a) contains surface-increasing elements (2) in the upper part and has slide valves (3) and melt discharge means (4) at the bottom of the reaction container, and a feed means with preheater (5) for nitrogen is arranged on the reaction container (1 a) and a discharge means with pipeline (6) is arranged in the upper region, a processing system being arranged downstream of said discharge means.
 17. The device as claimed in claim 16, wherein the reaction container (1 a) contains a separator having passages (17) and additionally surface-increasing elements (2) in the upper part.
 18. The device as claimed in claim 16 wherein the reaction container (1 a) contains a melt distributor (16) and at least two separator means arranged one on top of the other and having passages (17) and each having at least one overflow pipe (18) and containers (19) open at the top.
 19. The device as claimed in claim 16, wherein the feed means with preheater (5) for nitrogen is arranged above the level of the melt phase present in the lower part of the reaction container (1 a) and below the separator having passages (17).
 20. The device as claimed in claim 16, wherein, in in the reaction container (1 a) through which molten polyamide 6 flows, the discharge means for caprolactam is a jacketed pipeline (5) and is arranged in the upper region of the reaction container (1 a), at least above the melt level present above a separator having passages (17).
 21. The device as claimed in claim 16, wherein up to 8 reaction containers (1 a, 1 b, 1 c . . . ) are connected to one another in series in a plurality of stages.
 22. The device as claimed in claim 19, wherein a feed means with preheater (5) for dry nitrogen is arranged at least on the reaction container (1 c) of the last stage and there is a series connection with the upstream reaction containers (1 b, 1 a) for the nitrogen feed.
 23. The device as claimed in claim 20, wherein the jacketed pipeline (6) opens into the upper part of the collecting container (7) and, from the outlet of the collecting container (7), the jacketed pipeline (8) is connected via a pump (9) and a pressure valve (13) to a pipeline (6) in the vicinity of the caprolactam discharge means of the reaction container (1 a, 1 b, 1 c).
 24. The device as claimed in claim 20, wherein a line which connects the collecting container (7) to a rectifier (10), heat exchanger (11) and an immersion tank (12) is arranged on the upper part of the collecting container (7).
 25. The device as claimed in claim 20, wherein, at the outlet of the collecting container (7) and after the pump (9), a pipeline is led via the control valve (14) to a polymerization reactor.
 26. The device as claimed in claim 16, wherein the melt discharge means (4) of the last reaction stage is connected to the heat exchanger (17).
 27. The process is claimed in claim 1 in which the melt is a melt of polyamide
 6. 